Zilebesiran, an RNA Interference Therapeutic Agent for Hypertension.

BACKGROUND: Angiotensinogen is the sole precursor of angiotensin peptides and has a key role in the pathogenesis of hypertension. Zilebesiran, an investigational RNA interference therapeutic agent with a prolonged duration of action, inhibits hepatic angiotensinogen synthesis. METHODS: In this phase 1 study, patients with hypertension were randomly assigned in a 2:1 ratio to receive either a single ascending subcutaneous dose of zilebesiran (10, 25, 50, 100, 200, 400, or 800 mg) or placebo and were followed for 24 weeks (Part A). Part B assessed the effect of the 800-mg dose of zilebesiran on blood pressure under low- or high-salt diet conditions, and Part E the effect of that dose when coadministered with irbesartan. End points included safety, pharmacokinetic and pharmacodynamic characteristics, and the change from baseline in systolic and diastolic blood pressure, as measured by 24-hour ambulatory blood-pressure monitoring. RESULTS: Of 107 patients enrolled, 5 had mild, transient injection-site reactions. There were no reports of hypotension, hyperkalemia, or worsening of renal function resulting in medical intervention. In Part A, patients receiving zilebesiran had decreases in serum angiotensinogen levels that were correlated with the administered dose (r = -0.56 at week 8; 95% confidence interval, -0.69 to -0.39). Single doses of zilebesiran (≥200 mg) were associated with decreases in systolic blood pressure (>10 mm Hg) and diastolic blood pressure (>5 mm Hg) by week 8; these changes were consistent throughout the diurnal cycle and were sustained at 24 weeks. Results from Parts B and E were consistent with attenuation of the effect on blood pressure by a high-salt diet and with an augmented effect through coadministration with irbesartan, respectively. CONCLUSIONS: Dose-dependent decreases in serum angiotensinogen levels and 24-hour ambulatory blood pressure were sustained for up to 24 weeks after a single subcutaneous dose of zilebesiran of 200 mg or more; mild injection-site reactions were observed. (Funded by Alnylam Pharmaceuticals; ClinicalTrials.gov number, NCT03934307; EudraCT number, 2019-000129-39.).

Evaluation of RNAi therapeutics VIR-2218 and ALN-HBV for chronic hepatitis B: Results from randomized clinical trials.

BACKGROUND & AIMS: Current therapy for chronic hepatitis B virus (cHBV) infection involves lifelong treatment. New treatments that enable HBV functional cure would represent a clinically meaningful advance. ALN-HBV and VIR-2218 are investigational RNA interference therapeutics that target all major HBV transcripts. METHODS: We report on: i) the safety of single doses of VIR-2218 (modified from ALN-HBV by enhanced stabilization chemistry plus technology to reduce off-target, seed-mediated binding while maintaining on-target antiviral activity) and ALN-HBV in humanized mice; ii) a cross-study comparison of the safety of single doses of VIR-2218 and ALN-HBV in healthy human volunteers (n = 24 and n = 49, respectively); and iii) the antiviral activity of two doses of 20, 50, 100, 200 mg of VIR-2218 (total n = 24) vs. placebo (n = 8), given 4 weeks apart, in participants with cHBV infection. RESULTS: In humanized mice, alanine aminotransferase (ALT) levels were markedly lower following administration of VIR-2218 compared with ALN-HBV. In healthy volunteers, post-treatment ALT elevations occurred in 28% of participants receiving ALN-HBV compared with none in those receiving VIR-2218. In participants with cHBV infection, VIR-2218 was associated with dose-dependent reductions in hepatitis B surface antigen (HBsAg). The greatest mean reduction of HBsAg at Week 20 in participants receiving 200 mg was 1.65 log IU/ml. The HBsAg reduction was maintained at 0.87 log IU/ml at Week 48. No participants had serum HBsAg loss or hepatitis B surface antibody seroconversion. CONCLUSIONS: VIR-2218 demonstrated an encouraging hepatic safety profile in preclinical and clinical studies as well as dose-dependent HBsAg reductions in patients with cHBV infection. These data support future studies with VIR-2218 as part of combination regimens with a goal of HBV functional cure. TRIAL REGISTRATION: ClinicalTrials.gov identifiers: NCT02826018 and NCT03672188. IMPACT AND IMPLICATIONS: A significant unmet need exists for therapies for chronic HBV (cHBV) infection that achieve functional cure. We report clinical and non-clinical data on two investigational small-interfering RNAs that target HBx, ALN-HBV and VIR-2218, demonstrating that incorporation of enhanced stabilization chemistry plus technology in VIR-2218 reduces its propensity to cause ALT elevations relative to its parent compound, ALN-HBV. We also show that VIR-2218 reduces hepatitis B surface antigen levels in a dose-dependent manner in participants with cHBV infection. These studies support the continued development of VIR-2218 as part of therapeutic regimens for cHBV infection, with the goal of a functional cure, and are important for HBV researchers and physicians.

Thyroid Nodules in Pediatric Patients: Sonographic Characteristics and Likelihood of Cancer.

Purpose To determine the relationship between demographic and sonographic characteristics of thyroid nodules and malignancy in a pediatric population. Materials and Methods All thyroid nodules in patients younger than 19 years that underwent ultrasound (US)-guided fine-needle aspiration biopsy between January 2004 and July 2017 were retrospectively identified. Age, sex, and background appearance of the thyroid gland were recorded for each patient, and sonographic characteristics and pathologic diagnosis were recorded for each nodule. Demographic and sonographic characteristics were assessed to determine which were associated with malignancy. Categorical and continuous variables and interobserver variability were assessed. Results A total of 404 nodules in 314 patients (82.8% female) (age range, 2-18 years; mean age, 14.9 years) were analyzed. A total of 77 nodules (19.1%) were malignant, the majority of which were papillary thyroid carcinoma (n = 68 [88.3%]). The likelihood of malignancy did not differ between boys and girls (27.8% vs 22.7%, P = .64), nor did it differ between prepubertal and pubertal patients (18.8% vs 19.1%, P > .99). The cancer rate in patients with a solitary nodule was higher than that in patients with multiple nodules (29.4% vs 14.2%, P = .003). Sonographic characteristics associated with malignant nodules included larger size, solid parenchyma, taller-than-wide shape, presence of speckled calcifications, lack of a smooth margin, and presence of abnormal lymph nodes. Interobserver variability for assessment of sonographic characteristics ranged from moderate to very strong. Conclusion In children with thyroid nodules, solitary nodules, larger nodule size, solid parenchyma, taller-than-wide shape, speckled calcifications, irregular margins, and abnormal lymph nodes raise concern for malignancy.

Ectopic Intrathyroidal Thymic Tissue Mimicking Thyroid Nodules in Children.

Ectopic intrathyroidal thymic tissue is a benign lesion of nonthyroid origin occasionally found in the pediatric thyroid gland. Accurate diagnosis of such lesions is critical to avoid unnecessary biopsy or surgery. Twelve children referred to our center for the concern of thyroid nodules were found to have intrathyroidal thymic tissue. Most of the lesions had a classic sonographic appearance of a hypoechoic mass with sharp margins and multiple focal internal nonshadowing echogenicities identical to thymic tissue. Sonography and, in select cases, fine-needle aspiration can be used to diagnose benign thymic tissue within the thyroid and avoid unnecessary surgery.

Prevalence and Significance of Thyroglobulin Antibodies in Pediatric Thyroid Cancer.

Context: Circulating thyroglobulin antibodies (TgAb) can confound measurement of serum thyroglobulin and impair thyroid cancer surveillance. Few data exist on the significance of TgAb in pediatric thyroid cancer. Objective: To describe the prevalence, natural history, and clinical significance of TgAb in children with thyroid cancer. Design: Retrospective cohort study. Setting: Single academic pediatric center. Patients: Seventy-three consecutive children (≤18 years) with nonmedullary thyroid cancer who had serum TgAb measured within 6 months after diagnosis. Main Outcome Measures: Prevalence and natural history of TgAb; association of TgAb status and resolution with patient and disease characteristics. Results: TgAb were detected in 41% of subjects (30 of 73) and were associated with lymph node metastasis (83% vs 53%, P = 0.01) but not distant metastasis. In patients with TgAb, resolution occurred in 44% (11 of 25) over a median follow-up of 3.8 years. Median time to clear TgAb was 10.7 months, and 10 of 11 patients who cleared (91%) did so within 2 years. Resolution of TgAb was associated with lower initial TgAb level (median 4.5 vs 76 normalized units, P = 0.003). TgAb positivity at diagnosis was not independently associated with persistent or recurrent disease (odds ratio 3.20, 95% confidence interval 0.95 to 10.80, P = 0.06). Conclusions: TgAb are common at diagnosis in children with thyroid cancer but resolve in nearly half of patients within 1 to 2 years. TgAb are associated with the presence of lymph node metastasis at diagnosis, but the long-term prognostic significance remains to be determined.

Myocardial Induction of Type 3 Deiodinase in Dilated Cardiomyopathy.

BACKGROUND: The thyroid hormone-inactivating enzyme type 3 deiodinase (D3) is induced during hypertrophic and ischemic cardiomyopathy, leading to a state of local cardiac hypothyroidism. Whether D3 induction occurs in dilated cardiomyopathy is unknown. METHODS: This study characterized changes in cardiac D3 and thyroid hormone signaling in a transgenic model of progressive dilated cardiomyopathy (TG9 mice). RESULTS: Cardiac D3 was dramatically induced 15-fold during the progression of dilated cardiomyopathy in TG9 mice. This D3 induction localized to cardiomyocytes and was associated with a decrease in myocardial thyroid hormone signaling. CONCLUSIONS: Cardiac D3 is induced in a mouse model of dilated cardiomyopathy, indicating that D3 induction may be a general response to diverse forms of cardiomyopathy.

Features and Outcome of Autonomous Thyroid Nodules in Children: 31 Consecutive Patients Seen at a Single Center.

CONTEXT: Most thyroid nodules are benign and their accurate identification can avoid unnecessary procedures. In adult patients, documentation of nodule autonomy is accepted as reassurance of benign histology and as justification to forgo biopsy or thyroidectomy. In contrast, the negative predictive value of nodule autonomy in children is uncertain. Some recent publications recommend surgical resection as initial management, but few address the degree of TSH suppression or the specific scintigraphic criteria used to diagnose autonomy. OBJECTIVE: The objective of the study was to study the presenting features and cancer risk of children with autonomous nodules. DESIGN AND SETTING: Medical records of all 31 children diagnosed with autonomous nodules at our center from 2003 to 2014 were retrospectively reviewed. PATIENTS AND RESULTS: All children met full diagnostic criteria for autonomous nodules, defined by both autonomous 123I uptake into the nodule and the suppression of uptake in the normal thyroid parenchyma on scintigraphy performed during hypothyrotropinemia. The median age of presentation was 15 years (range 3-18 y) with a female to male ratio of 15:1. Fifty-eight percent of patients had solitary nodules and 42% had multiple nodules. The median size of each patient's largest autonomous nodule was 39 mm (range 18-67 mm). Most of the children in this series (68%) had diagnostic biopsies and/or operative pathology of their largest autonomous nodule, which showed benign cytology or histology in all cases. CONCLUSIONS: In this pediatric series, the cancer rate observed in biopsied or resected autonomous nodules was 0%. Whereas larger studies are needed to confirm our findings, these results agree with earlier reports suggesting that thyroid cancer is rare in rigorously defined autonomous nodules and support that conservative management may be offered to selected children who meet strict diagnostic criteria for autonomous nodules, deferring definitive therapies until adulthood when the risks of thyroidectomy and 131I ablation are lower.

eIF1A augments Ago2-mediated Dicer-independent miRNA biogenesis and RNA interference.

MicroRNA (miRNA) biogenesis and miRNA-guided RNA interference (RNAi) are essential for gene expression in eukaryotes. Here we report that translation initiation factor eIF1A directly interacts with Ago2 and promotes Ago2 activities in RNAi and miR-451 biogenesis. Biochemical and NMR analyses demonstrate that eIF1A binds to the MID domain of Ago2 and this interaction does not impair translation initiation. Alanine mutation of the Ago2-facing Lys56 in eIF1A impairs RNAi activities in human cells and zebrafish. The eIF1A-Ago2 assembly facilitates Dicer-independent biogenesis of miR-451, which mediates erythrocyte maturation. Human eIF1A (heIF1A), but not heIF1A(K56A), rescues the erythrocyte maturation delay in eif1axb knockdown zebrafish. Consistently, miR-451 partly compensates erythrocyte maturation defects in zebrafish with eif1axb knockdown and eIF1A(K56A) expression, supporting a role of eIF1A in miRNA-451 biogenesis in this model. Our results suggest that eIF1A is a novel component of the Ago2-centred RNA-induced silencing complexes (RISCs) and augments Ago2-dependent RNAi and miRNA biogenesis.

Mice with hepatocyte-specific deficiency of type 3 deiodinase have intact liver regeneration and accelerated recovery from nonthyroidal illness after toxin-induced hepatonecrosis.

Type 3 deiodinase (D3), the physiologic inactivator of thyroid hormones, is induced during tissue injury and regeneration. This has led to the hypotheses that D3 impacts injury tolerance by reducing local T3 signaling and contributes to the fall in serum triiodothyronine (T3) observed in up to 75% of sick patients (termed the low T3 syndrome). Here we show that a novel mutant mouse with hepatocyte-specific D3 deficiency has normal local responses to toxin-induced hepatonecrosis, including normal degrees of tissue necrosis and intact regeneration, but accelerated systemic recovery from illness-induced hypothyroxinemia and hypotriiodothyroninemia, demonstrating that peripheral D3 expression is a key modulator of the low T3 syndrome.

Intrinsic expression of a multiexon type 3 deiodinase gene controls zebrafish embryo size.

Thyroid hormone is a master regulator of differentiation and growth, and its action is terminated by the enzymatic removal of an inner-ring iodine catalyzed by the selenoenzyme type 3 deiodinase (dio3). Our studies of the zebrafish reveal that the dio3 gene is duplicated in this species and that embryonic deiodination is an important determinant of embryo size. Although both dio3 paralogs encode enzymatically active proteins with high affinity for thyroid hormones, their anatomic patterns of expression are markedly divergent and only embryos with knockdown of dio3b, a biallelically expressed selenoenzyme expressed in the developing central nervous system, manifest severe thyroid hormone-dependent growth restriction at 72 hours post fertilization. This indicates that the embryonic deficiency of dio3, once considered only a placental enzyme, causes microsomia independently of placental physiology and raises the intriguing possibility that fetal abnormalities in human deiodination may present as intrauterine growth retardation. By mapping the gene structures and enzymatic properties of all four zebrafish deiodinases, we also identify dio3b as the first multiexon dio3 gene, containing a large intron separating its open reading frame from its selenocysteine insertion sequence (SECIS) element.

Thyroid hormone inactivation in gastrointestinal stromal tumors.

Gastrointestinal stromal tumors (GISTs) are resistant to traditional chemotherapy but are responsive to the tyrosine kinase inhibitors imatinib and sunitinib. The use of these agents has improved the outcome for patients but is associated with adverse effects, including hypothyroidism. Multiple mechanisms of this effect have been proposed, including decreased iodine organification and glandular capillary regression. Here we report the finding of consumptive hypothyroidism caused by marked overexpression of the thyroid hormone-inactivating enzyme type 3 iodothyronine deiodinase (D3) within the tumor. Affected patients warrant increased monitoring and may require supernormal thyroid hormone supplementation.

How are childhood thyroid nodules discovered: opportunities for improving early detection.

In a retrospective analysis of childhood thyroid nodules, 18% were radiographic incidentalomas and 41% were discovered by a clinician's palpation; 40% were discovered by patients' families. The latter group had the largest nodules and highest rates of thyroid cancer metastasis, suggesting opportunities for earlier detection through annual well-child visits.

A standardized assessment of thyroid nodules in children confirms higher cancer prevalence than in adults.

CONTEXT: Thyroid cancer is the most common endocrine malignancy, but due to its rare occurrence in the pediatric population, the cancer risk of childhood thyroid nodules is incompletely defined, and optimal management of children with suspected nodules is debated. OBJECTIVE: The aim was to study the presenting features and cancer risk of sporadic childhood thyroid nodules using a standardized clinical assessment and management plan. DESIGN AND SETTING: Boston Children's Hospital and Brigham and Women's Hospital collaborated to create a multidisciplinary pediatric thyroid nodule clinic and implement a standardized assessment plan. Upon referral for a suspected nodule, serum TSH was measured and hypothyrotropinemic patients underwent (123)I scintigraphy. All others underwent thyroid ultrasonography, and if this confirmed nodule(s) ≥ 1 cm, ultrasound-guided fine-needle aspiration was performed. Medical records were retrospectively reviewed and compared to a control population of 2582 adults evaluated by identical methods. PATIENTS AND RESULTS: Of 300 consecutive children referred for the initial evaluation of suspected thyroid nodules from 1997 to 2011, 17 were diagnosed with autonomous nodules by scintigraphy. Neck ultrasonography performed in the remainder revealed that biopsy was unnecessary in over half, either by documenting only sub-centimeter nodules or showing that no nodule was present. A total of 125 children met criteria for thyroid biopsy, which was performed without complication. Their rate of cancer was 22%, significantly higher than the adult rate of 14% (P = .02). CONCLUSIONS: Neck ultrasonography and biopsy were key to the evaluation of children with suspected thyroid nodules. Although the relative cancer prevalence of sonographically confirmed nodules ≥ 1 cm is higher in pediatric patients than adults, most children referred for suspected nodules have benign conditions, and efforts to avoid unnecessary surgery in this majority are warranted.

Simulation of post-thyroidectomy treatment alternatives for triiodothyronine or thyroxine replacement in pediatric thyroid cancer patients.

BACKGROUND: As in adults, thyroidectomy in pediatric patients with differentiated thyroid cancer is often followed by (131)I remnant ablation. A standard protocol is to give normalizing oral thyroxine (T(4)) or triiodothyronine (T(3)) after surgery and then withdraw it for 2 to 6 weeks. Thyroid remnants or metastases are treated most effectively when serum thyrotropin (TSH) is high, but prolonged withdrawals should be avoided to minimize hypothyroid morbidity. METHODS: A published feedback control system model of adult human thyroid hormone regulation was modified for children using pediatric T(4) kinetic data. The child model was developed from data for patients ranging from 3 to 9 years old. We simulated a range of T(4) and T(3) replacement protocols for children, exploring alternative regimens for minimizing the withdrawal period, while maintaining normal or suppressed TSH during replacement. The results are presented with the intent of providing a quantitative basis to guide further studies of pediatric treatment options. Replacement was simulated for up to 3 weeks post-thyroidectomy, followed by various withdrawal periods. T(4) vs. T(3) replacement, remnant size, dose size, and dose frequency were tested for effects on the time for TSH to reach 25 mU/L (withdrawal period). RESULTS: For both T(3) and T(4) replacement, higher doses were associated with longer withdrawal periods. T(3) replacement yielded shorter withdrawal periods than T(4) replacement (up to 3.5 days versus 7-10 days). Higher than normal serum T(3) concentrations were required to normalize or suppress TSH during T(3) monotherapy, but not T(4) monotherapy. Larger remnant sizes resulted in longer withdrawal periods if T(4) replacement was used, but had little effect for T(3) replacement. CONCLUSIONS: T(3) replacement yielded withdrawal periods about half those for T(4) replacement. Higher than normal hormone levels under T(3) monotherapy can be partially alleviated by more frequent, smaller doses (e.g., twice a day). LT(4) may be the preferred option for most children, given the convenience of single daily dosing and familiarity of pediatric endocrinologists with its administration. Remnant effects on withdrawal period highlight the importance of minimizing remnant size.

Thyroid surgery at Children's Hospital Boston: a 35-year single-institution experience.

BACKGROUND/PURPOSE: Thyroidectomy is the primary therapy for thyroid cancer and an established treatment of hyperthyroidism. Because of the relative rarity of these conditions in childhood, few single-institution series exist in the pediatric literature. Here we analyze our institution's experience to assess patient demographics, operative risks, and the role of preoperative testing. METHODS: This is a retrospective chart review of 175 consecutive patients not older than 18 years who underwent thyroid surgery at Children's Hospital Boston from 1970 to 2004. RESULTS: The most common indication for thyroidectomy was thyroid nodules (83%), followed by hyperthyroidism (7%) and goiter (7%). For children referred for nodules, we observed a peak incidence in adolescence and a female to male ratio of 3.7:1. Cancer was found in 36%, with papillary thyroid cancer the most common subtype (85%). Operative complications were rare, with permanent hypocalcemia in 2 (4.7%) of 43 patients who underwent bilateral resection for thyroid nodules (no cases of permanent hypocalcemia in other procedures). Permanent unilateral vocal cord paralysis was documented in 2 children after the resection of malignant nodules. CONCLUSIONS: Pediatric thyroidectomy can be performed with low operative risk. Because permanent hypocalcemia remains an obligate risk of bilateral thyroidectomy, we recommend the routine use of preoperative fine-needle aspiration to guide the extent of initial surgical resection, reserving near-total thyroidectomy for those cases where cytology is positive for malignancy.

Thyroid nodules and cancer in children with PTEN hamartoma tumor syndrome.

CONTEXT: Phosphatase and tensin homolog (PTEN) hamartoma tumor syndrome (PHTS) is a complex disorder caused by germline-inactivating mutations of the PTEN tumor suppressor gene. Carriers develop benign and malignant tumors of multiple tissues, including the breast, thyroid, intestine, and skin. Surveillance to facilitate the early detection and treatment of malignancies is recommended but, because thyroid cancers have been reported almost exclusively in adults, childhood risk is considered to be low, and consensus guidelines recommend that surveillance imaging begin at 18 yr of age. OBJECTIVE/PATIENTS: Seven children with PHTS referred to two thyroidologists form the basis of this report. Medical records, operative histology, and PTEN mutational analysis were reviewed to evaluate the pediatric presentation of PHTS-associated thyroid neoplasia. RESULTS: Five of the seven children presented with thyroid nodules or thyroid cancer between the ages of 6 and 12 yr, often as the initially identified component of their PHTS. Two others were diagnosed with PHTS on the basis of extrathyroidal features but had markedly abnormal screening ultrasounds with solid thyroid nodule(s) of at least 2 cm, despite the documentation of normal physical examinations. Five of the seven children in this cohort developed thyroid cancer. CONCLUSIONS: Patients with PHTS can develop thyroid nodules and thyroid cancer in early childhood. This argues both for a high index of suspicion for PHTS in children diagnosed with multiple thyroid nodules and for careful thyroid surveillance in children diagnosed with PHTS. Because early detection improves the outcome of thyroid cancer, we recommend ultrasound surveillance for all patients upon the confirmation of a germline PTEN mutation, regardless of their age.

Type 2 iodothyronine deiodinase levels are higher in slow-twitch than fast-twitch mouse skeletal muscle and are increased in hypothyroidism.

Because of its large mass, relatively high metabolic activity and responsiveness to thyroid hormone, skeletal muscle contributes significantly to energy expenditure. Despite the presence of mRNA encoding the type 2 iodothyronine-deiodinase (D2), an enzyme that activates T(4) to T3, very low or undetectable activity has been reported in muscle homogenates of adult humans and mice. With a modified D2 assay, using microsomal protein, overnight incubation and protein from D2 knockout mouse muscle as a tissue-specific blank, we examined slow- and fast-twitch mouse skeletal muscles for D2 activity and its response to physiological stimuli. D2 activity was detectable in all hind limb muscles of 8- to 12-wk old C57/BL6 mice. Interestingly, it was higher in the slow-twitch soleus than in fast-twitch muscles (0.40 ± 0.06 vs. 0.076 ± 0.01 fmol/min · mg microsomal protein, respectively, P < 0.001). These levels are greater than those previously reported. Hypothyroidism caused a 40% (P < 0.01) and 300% (P < 0.001) increase in D2 activity after 4 and 8 wk treatment with antithyroid drugs, respectively, with no changes in D2 mRNA. Neither D2 mRNA nor activity increased after an overnight 4 C exposure despite a 10-fold increase in D2 activity in brown adipose tissue in the same mice. The magnitude of the activity, the fiber specificity, and the robust posttranslational response to hypothyroidism argue for a more important role for D2-generated T(3) in skeletal muscle physiology than previously assumed.

Soy protein diet alters expression of hepatic genes regulating fatty acid and thyroid hormone metabolism in the male rat.

We hypothesized that consumption of soy protein isolate (SPI) or the soy isoflavone genistein (GEN) would modulate mRNA expression of genes underlying lipid and thyroid hormone metabolism in livers and small intestines of young adult male Sprague-Dawley rats. Early pregnant rat dams were placed on AIN-93G diets containing casein (CAS, control protein), SPI, or CAS+GEN. Litters were weaned to the same diet as their dam. SPI-fed (but not GEN-fed) male rats of 48 days of age had significant reductions in body weight, abdominal fat pad weight and hepatic content of lipid droplets and triglycerides. Hepatic peroxisome proliferator-activated receptor α (Ppara) transcripts were elevated with SPI but not GEN diet. Hepatic pyruvate dehydrogenase kinase-4 (Pdk4) and cytochrome P450 4A10 (Cyp4a10) mRNA abundance was reduced with SPI; the SPI effect on Cyp4a10 was recapitulated by GEN diet. SPI (but not GEN) suppressed Pdk4 and 3-hydroxy-3-methylglutaryl-CoA synthase 2 (Hmgcs2) mRNA abundance in duodenum. Liver iodothyronine deiodinase types 1 and 2 (Dio1 and Dio2) mRNA levels were increased with SPI diet; the effect on Dio2, but not Dio1 mRNAs, also was observed with GEN. SPI and GEN increased hepatic types 1 and 2 iodothyronine deiodinase (D1 and D2) activities. Effects of SPI and GEN on the above gene expression may contribute to the observed reductions in body and adipose tissue weight and liver lipid content in this model. Identification of the regulation, by genistein and soy protein, of iodothyronine deiodinase synthesis has potential applications for treatment and prevention of fatty liver disease and obesity.